1. Technical Field
Embodiments disclosed herein relate to an apparatus for and method of protecting a reticle in a lithography system from particle deposition using thermophoresis, but still allowing a high vacuum area for the photo-optics.
2. Related art
Reticles in a conventional lithographic system are typically protected by a clear faceplate, called a pellicle, to cover and protect the pattern side of a reticle. Increases in circuit densities on wafers result in the need for shorter wavelengths, e.g., λ≈11-14 nm (nominally 13 nm). With the shorter wavelength radiation, the pellicle cannot be utilized, as present materials absorb too much of the radiation for process efficiency and/or deteriorate quickly. Therefore, protecting the pattern side of a “pellicle-less” reticle from contamination while mounted on a reticle stage in a lithography system becomes important.
One alternative to a pellicle may be using a thermophoretic force, which relies on gas pressure and temperature gradients within the gas, to move particles from higher to lower temperature gas. The prior art describes at least one application of thermophoresis to protect lithography components such as wafers and reticles. A smaller enclosure around the reticle, which is maintained at a higher pressure than the vacuum environment in the larger enclosure surrounding the smaller enclosure can provide thermophoretic protection. In the prior art, the wall of the smaller enclosure opposite the downward facing, horizontal surface of the reticle has an aperture through which the gas flows out.
In the prior art, when elevated pressure is maintained over the surface to be protected, the portion of the reticle to be exposed to a beam of radiation for use in wafer processing may not be presented in the high-vacuum environment desired for optimum use of short wavelength radiation. Accordingly, an alternative apparatus and approach for providing thermophoretic particle protection to a reticle while in use in a high vacuum environment is needed.